@usuba dashi.
I have that problem too with recipes that simply call for Kosher Salt like brines. The Diamond and Morton brand have a different weight and texture. I'm not sure, but I think the default is the Diamond brand. I seem to remember the crystals on diamond brand were larger.

I only use course sea salt and only sea salt from Celtic Salt company in Ashville, NC, They have the real sel de gris. . . . . very coarse. Fish should only be seasoned with sea salt, not Kosher. Save Kosher for land animals.

Diamond's salt is proper evaporated flat crystals . . Morton uses a roller to make their flat salt. . . . is not the same. David's is even better.

possible that skin of fish was wet and used mortons (gasp!). Also wonder if my salt mixture was too "wet" (eg: not enough salt for egg whites)....Seems like sle de gris would change the investment of this dramatically. In NYC a whole fish aint cheap plus premium salt?